1. Field of the Invention
The present invention relates to an electrophotographic color image forming apparatus having a plurality of photosensitive bodies, and more particular, to a technique of detecting misregistering of respective color toner images formed on the photosensitive bodies and precisely registering the images on a recording medium to make transfer.
2. Description of the Related Art
Conventionally, in a color image forming apparatus employing an electrophotographic technique, a photosensitive body serving as an image carrying body is charged by electrifying means, light is irradiated to form a latent image on the charged photographic body in accordance with image information, the image is developed by a developing means, and the developed toner image is transferred to a sheet material or the like to form an image.
Meanwhile, with the spread of color images, a tandem system color image forming apparatus has been proposed for forming a full-color image by preparing a plurality of image stations, in which such a series of image forming processes are performed, forming respective images of cyan image, magenta image, yellow image and preferably black image as well on respective image carrying bodies, and transferring respective color images to a sheet material at respective transfer positions of the respective image carrying bodies in superposing manner. Such tandem type color image forming apparatus is advantageous in attainment of high speed operation since it includes respective image forming units for respective color images.
Such color image forming apparatus, however, offers a problem in how to favorably execute registering of respective images formed in different image forming units. This is because misregistration of image forming positions for four color images transferred to a sheet or the like eventually leads to positional misregistration or as a change in color tone.
Meanwhile, misregistering of transferred images includes, as shown in FIGS. 17A to 17E, misregistering in a moving direction (a direction of an arrow A in FIG. 17A) of a transfer material (herein-after, referred to as "subscanning misregistering"; see FIG. 17A), misregistering in a scanning direction (perpendicular to the direction of an arrow A in FIG. 17B) (hereinafter, referred to as "primary scanning misregistering"; see FIG. 17B), misregistering in a skew direction (hereinafter, referred to as "skew misregistering"; see FIG. 17C), differences in magnification error (see FIG. 17D) and differences in curvature error (see FIG. 17E). Actually, the misregistering of transferred images amounts to superposed misregistering and differences of the above five kinds.
In the case of subscanning misregistering in FIG. 17A, the main cause for misregistering results from misregistering in mounting of respective image stations or scanning optical systems and in mounting of a lens or a mirror (not shown) in a scanning optical system. This is the same with the case of primary scanning misregistering in FIG. 17B.
Also, the main cause for skew misregistering in the skew direction in FIG. 17C results from angular misregistering of rotating shafts on photosensitive drums in the image stations and angular misregistering of scanning optical systems. The main cause for misregistering due to differences in magnification error shown in FIG. 17D results from differences in scanning length due to errors in optical length from respective scanning optical systems to the photosensitive drums in the image stations. The main cause for misregistering due to differences in curvature error in FIG. 17E results from errors in assembling of lenses or the like in the respective scanning optical systems.
To correct these five kinds of misregistering or differences, it has been proposed to depict a reference pattern (hereinafter, referred to as "registering pattern") in advance, to detect this by means of a plurality of sensors (misregistering detection), to calculate an amount of misregistering from the detected result and to register respective images (correction of misregistering) in accordance with the calculated amount of misregistering.
Hereinafter, a conventional detection of a registering pattern and a correcting operation of misregistering will be described.
FIG. 18 is an illustration showing a construction of a conventional registering pattern detecting means (hereinafter, referred to as "pattern detecting means"). FIG. 19 is an illustration showing an arrangement of a conventional registering pattern on an intermediate transfer belt and conventional pattern detecting means. FIGS. 20 and 21 are illustrations showing an arrangement of conventional registering patterns on an intermediate transfer belt and conventional pattern detecting means and output signals of the pattern detecting means.
As shown in FIG. 18, the pattern detecting means 40 comprises an image sensor (hereinafter, referred to as "CCD") 41, a light source such as a lamp 42 and a cell focussing lens array 43 for focussing reflected light on the CCD 41. Such pattern detecting means 40a and 40b are so disposed that picture elements in the CCDs 41a and 41b arranged in a line are positioned on a line perpendicular to a conveying direction A of the intermediate transfer belt 12 as shown in FIG. 19. As shown in the drawings, two detecting means in total are disposed one near each of two ends of the intermediate transfer belt 12 in a widthwise direction perpendicular to the conveying direction A.
With the above arrangement, detecting/correcting actions of registering patterns comprises forming predetermined registering patterns such as straight lines, figures or the like (e.g., toner images 44, 45, 46 and 47 for respective colors at predetermined spacings on a line perpendicular to the conveying direction A of the intermediate transfer belt 12) and measuring misregistering of respective colors (misregistering) by means of the pattern detecting means 40a and 40b as shown in FIG. 19.
Hereupon, as shown in FIG. 20(a), subscanning misregistering shown in FIG. 17A can be obtained by calculating misregistering for respective colors (.DELTA.Y1=.DELTA.T1.multidot.v) from time lags (.DELTA.T1=T-T1, T is a predetermined design value) between points of time, at which registering patterns 44, 45, 46 and 47 of respective colors on the intermediate transfer belt 12 pass the CCD 41a in the pattern detecting means and predetermined design values and from the conveying speed v of the intermediate transfer belt 12.
As shown in FIG. 21A, the primary scanning misregistering shown in FIG. 17B can be obtained by calculating misregistering for respective colors from differences of positions of those picture elements (.DELTA.X1) when scanning start positions of registering patterns 44, 45, 46 and 47 of respective colors on the intermediate transfer belt 12 pass the CCD 41a in the pattern detecting means.
As shown in FIG. 20(b), the skew error shown in FIG. 17C can be obtained by calculating skew errors (.DELTA.Y2=.DELTA.T2.multidot.v) of respective colors from time lags (.DELTA.T2) between points of time, at which the registering patterns 44, 45, 46 and 47 of the same color formed on both widthwise sides of the intermediate transfer belt 12 pass the CCD 41a and CCD 41b in the pattern detecting means, and from the conveying speed v of the intermediate transfer belt 12.
As shown in FIGS. 21(a) and (b), the magnification error shown in FIG. 17D can be obtained by calculating magnification errors (.DELTA.X3=.DELTA.X2-.DELTA.X1) of respective colors from differences (.DELTA.X2, .DELTA.X1) of positions of those picture elements when scanning start positions and scanning terminating positions of registering patterns 44, 45, 46 and 47 of the same color on the intermediate transfer belt 12 pass the CCD 41a and CCD41b in the respective pattern detecting means.
Thus, misregistering correcting operation is performed on the basis of amounts of the four kinds of misregistering calculated.
Here, for the subscanning misregistering shown in FIG. 17A and the primary scanning misregistering shown in FIG. 17B, amounts of misregistering are corrected by adjusting scanning timings for the respective colors (not shown).
Also, for the skew error shown in FIG. 17C and the magnification error shown in FIG. 17D, amounts of misregistering are corrected by adjusting the optical systems in the exposure means (not shown) by means of actuators (not shown) (not shown).
The curvature error shown in FIG. 17E is tackled by enhancing assembling accuracy of lenses or the like in the exposure means (not shown) and not corrected since the error cannot be correctly measured.
The amounts of misregistering of four colors are detected with the above arrangement and in the above actions to make correction corresponding to the amounts of misregistering.
With such conventional technique as mentioned above, however, it is difficult to obtain an inexpensive color image forming apparatus because expensive CCDs are used in detection of registering pattern.
Further, CCDs for detecting registering patterns require a predetermined period of storage time in order to secure its detecting accuracy. Since the storage time must be necessarily shortened to read a registering pattern on an intermediate transfer belt moving at high speed in a high-speed color image forming apparatus, there were caused a problem in an increased cost because of the need for a raised luminosity of a light source so as to increase amount of light in pattern detecting means and for CCDs of high accuracy employed for securing detecting precision.
Further, as for the storage time in a high speed color image forming apparatus, a predetermined given storage time can be secured by making the moving speed of the intermediate transfer belt lower than that in the normal printing operation, but even if a registering pattern is detected in such a manner, there is caused a problem in that it is impossible to correctly detect misregistering in a normal printing condition.
Further, in a color image forming apparatus using the intermediate transfer process, a registering pattern must be formed on the intermediate transfer belt. Since the intermediate transfer belt must contain carbon by nature, its color is generally black. For this reason, when a registering pattern made of a black toner image on the black intermediate transfer belt is detected, there is caused a problem in that the detecting accuracy was very low since there is provided little difference between the intermediate transfer belt and the registering pattern.
Accordingly, it is one object of the present invention to provide an inexpensive color image forming apparatus capable of detecting a registering pattern at high speed and with high precision and making correction accurately.